1. Field of Invention
The present invention relates to a voltage-controlled oscillator (VCO) and more particularly to a calibration circuit for the VCO, suitable for calibrating the oscillation frequency of the VCO.
2. Description of Related Art
Referring to FIG. 1, it depicts a conventional frequency synthesizer for the voltage-controlled oscillator (VCO), having the adjustment function. As shown in the drawing, the frequency synthesizer comprises a VCO 101, a programmable control frequency divider 103, a phase detector 105, a charge pump 109 and a loop filter 111. The phase detector 105 simultaneously receives a reference frequency 107 and an output signal from the programmable control frequency divider 103, computes the reference frequency 107 and the output signal for obtaining a result, and transfers the result to the loop filter 111. The loop filter 111 transfers the result to the VCO 101.
Moreover, the frequency synthesizer further comprises a compensation unit 113 having a local oscillator 115, a combiner 117, and a low-pass filter 119. The compensation unit 113 receives the oscillation frequency output from the VCO 101 and outputs the signal to the programmable control frequency divider 103.
In the conventional art, the frequency synthesizer further comprises a replica charge pump 121 used for providing a test signal required in the calibration of the oscillation frequency. The replica charge pump 121 is under the control of a micro control unit 123. The micro control unit 123 contains the calibration action and data required in the calibration of the frequency. Furthermore, a frequency measurement unit 137 in the frequency synthesizer receives the reference frequency 107 and the signal output from the programmable control frequency divider 103 to provide the frequency indication signal to the micro control unit 123 for frequency calibration.
Additionally, the frequency synthesizer further has an adjuster unit 125 for the VCO inside. The adjuster unit 125 for the VCO comprises a sense amplifier 127, an analog-to-digital converter 129, a temperature sensor 133, a selector 131, and a capacitance storage unit 135. The analog-to-digital converter 129 is used to provide a digital signal to the micro control unit 123 to adjust the range of the oscillation frequency and to set the output signal of the programmable control frequency divider 103 close to the predetermined oscillation frequency as much as possible.
In the conventional technology described above, the frequency synthesizer has the function of auto-adjusting the oscillation frequency, but the design of the circuit is rather complex and each of the control units further comprises a plurality of control devices, which causes an increase of the manufacturing cost of the circuit and the difficulty in the design of the circuit.